Electrical resistance welding method and apparatus for same; and improved apparatus for testing welds

ABSTRACT

In an electric resistance welding machine for welding structural members together along a common region of contact through the intermittent application of heat generating electrical energy to said region of contact the combination of; electrode means adapted to releasably contact and clamp said members together so as to exert an external force to said members for urging said members together at said regions of contact, and to transmit and apply said electrical energy to said members in said region of contact on an intermittent basis, transformer means associated with said electrode means for supplying said electrical energy to be intermittently applied to said members in said region of contact, control means for controlling the intermittent application of said electrical energy to said region of contact for a selected timed interval, including means for interrupting said application of said electrical energy for a selected timed interval so as to present a series of intermittent impulses of said electrical energy adapted to incrementally raise the temperature of said members in said region of contact in stages so as to minimize the escape of said heat energy from said members in said region of contact and effectively weld said members together at said region of contact, said control means including first timing means for activating said application of said several impulses of electrical energy for a limited period at preset selected timed intervals, and second timing means for activating said interrupting means for a limited period at preset selected timed intervals.

FIELD OF THE INVENTION

This is a division of application Ser. No. 179,634 filed Aug. 20, 1980.

This invention relates to improvements in a method for weldingstructural components together by fusion and particularly by means of anovel application of electrical resistance welding procedures.

More particularly, this invention relates to improvements in thefabrication of open web structural steel joists or trusses using theimproved welding method and includes improvements in equipment forautomatically implementing and controlling the sequential steps employedin fabricating the open web structures and in the testing of theadequacy of the welds.

BACKGROUND OF THE INVENTION

Joists or trusses include at least two chord or chord-like membersconnected by suitable load distributing lattices or webs. A typicaljoist or truss consists of two parallel chord members joined by anintermediate supporting web which may comprise a solid section or apattern of struts designed to ensure that such structure has requisiteload bearing capabilities.

A typical web of the type under consideration takes the form of asuitable structural steel rod bent upon itself into an undulating orzig-zag configuration of generally uniform pattern with the respectiveapices presented by the bent rod secured or connected to the spacedupper and lower chord members which members are usually arranged inparallel relation.

When such structure is fabricated out of steel and used to support theflooring or carry ceiling components it is commonly referred to as an"open-web" steel joist.

A typical method of attaching or connecting chord members to the webapices is by means of electrical resistance welding.

Electrical resistance welding is a process whereby fushion is producedby the heat generated from the resistance of the metal components to bejoined together to the flow of electrical current when contacted andclamped between appropriate electrodes within an electrical circuit. Noexternal heat source is required in order to achieve fusion, nor are anyfluxes or filler materials necessary.

The current for resistance welding is normally supplied through atransformer which transforms the high voltage, low amperage power supplyto usable high amperage current at low voltages. The pressure for theelectrode clamping forces is generated by a suitable hydraulic system.

In any electrical conductor, current flow creates heat. The amount ofheat (H) generated depends upon the amount of current (I), theresistance of the conductor (R) and the time (T) during which thecurrent is flowing. Heat generated in resistance welding can beexpressed in the following manner:

    H=I.sub.2 RT

H=heat generated in joules

I=current in rms amps

R=resistance of the work in ohms

T=time of current flow in seconds

This formula indicates that the heat generated is directly proportionalto the square of the welding current and to the resistance and to thetime of current flow. The heat generated in such a procedure is used inpart to fuse the chord and web members together in the regions ofcontact but with some of their useful heat being lost by conduction tothe work pieces themselves and to the contacting clamping electrodes.

Radiation loss to the atmosphere is relatively insignificant. The heatlost by conduction from the weld zone to the work pieces and to thecontacting electrodes is directly proportional to the temperaturedifferential existing between them. The greater the temperaturedifferential therefore the greater the heat loss. This heat loss notonly produces discoloration, warpage or twisting but undesirablemetallurgical changes in the work pieces in the form of large graingrowth in the region immediately surrounding the region of contactcommonly designated the weld zone or the "nugget".

Large grain growth is undesirable because it produces weaknesses andbrittleness at the weld joints and cause failure.

It follows that by controlling heat lost or dissipation by conduction,discoloration, warpage or twisting and large grain growth can bereduced. Further it is apparent that by lowering the temperaturedifferential the greater will be the tendency to localize or confine thegenerated heat to the weld zone or "nugget", and therefore the escape ofsaid generated heat energy from said region of contact will beminimized.

According to the formula, resistance is a critical factor in thegeneration of heat.

In making a weld, the current is passed from one electrode through thebase metal in the work pieces to the other electrode. During thispassage the current encounters several resistance zones including:

(a) the electrical resistance of each electrode;

(b) the contact resistance between each electrode and the base metal,the magnitude of which depends on the surface condition of the basemetal and electrode, the size and contour of the electrode face, and theelectrode clamping force;

(c) the resistance of each piece of the base metal which is directlyproportional to the resistivity of the base metal and its thickness andinversely proportional to the cross-sectional area of the current path;

(d) the base metal interface where the weld formation starts, which isthe point of highest resistance and therefore the point of greatest heatgeneration.

Should the surface of the base metal present oxides or scale then theresistance of the base metal would be correspondingly higher than if thebase metal was free of same. Therefore the heat generated in the weldzone would be higher than normal and concomitantly the temperaturedifferential between the weld zone and the surrounding metal would begreater than normal which results in undesirable grain growth asmentioned above. As well, the scale tends to impede the conduction ofheat between the two work pieces being joined, and presents an uncleansurface for welding which results in sparking during the weldingoperation and ultimately produces an unacceptable joint or connection.Accordingly, each work piece of base metal should be kept substantiallyfree of scale.

Also, for best results the electrodes should be kept substantially freeof the oxide coatings.

In making a satisfactory weld there is one factor not always given dueconsideration because of the difficulty in accurately predicting theprecise effect. This factor is correct heat balance which is thecondition in which the fusion zone in each work piece to be joinedundergoes approximately the same degree of heating. The quality of theweld is enhanced when each work piece to be joined experiences the samedegree of heating.

As mentioned above the resistance of each work piece is inter aliainversely proportional to the cross-sectional area of the current path.Therefore when resistance welding two equal cross-sectional areas of thesame metal the heat balance is generally automatic. However, joists ortrusses typically have disproportionate thicknesses or cross-sectionalareas of chord members and web rods and therefore the heat balance isnot automatic as the cross-sectional area in the common region ofcontact heat up at different rates. In welding dissimilarcross-sectional areas, a longer period of current flow or heat cycle isrequired to provide a more uniform distribution of heat throughout theasymmetrical resistance path extending between the clamping electrodes.

A number of pieces of equipment and proposals have heretofore beendeveloped and used for welding the chord components of a joist or atruss to the web on an assembly line basis so as to reduce the cost ofsame. The quality however of the welds of the finished joists or trusseshas also given some concern.

U.S. Pat. No. 3,158,731 discloses apparatus for automaticallyfabricating a truss where the chords are continuously formed in rollerdies from coiled strip material while web wire also from a coil isstraightened then bent into a zig-zag shape whereupon all threecomponents are assembled together then resistance welded together andfinally the truss so produced is automatically cut into lengths andpainted.

Another alternative is illustrated in U.S. Pat. No. 3,228,977 whichrelates to a welding device which automatically moves the latticingstrips in a step-by-step manner, with pauses between the motion forprofiling the latticing into a zig-zag configuration and for welding thelatticing to the chords. This patent further discloses testing equipmentwhich comprises hydraulic cylinders and grippers which will apply aforce F to one of the welded joints on the upper chord, while a forceF/2 is applied to each of the adjacent two welded joints on the lowerchord.

Still another alternative is disclosed by U.S. Pat. No. 3,427,699 whichrelates to a production line operation wherein chords of predeterminedcross-section are stacked at two separate stations and in parallelrelation and individually fed forward from the bottom of the stack, thensheared to length and aligned with a web of zig-zag configuration. Thechords and web are then fed forwardly together in continuous alignmentand welded together.

A further alternative is illustrated in U.S. Pat. No. 3,487,861 whichdiscloses apparatus providing means for performing the steps ofsimultaneously supplying a pair of straight side wires and anintermediate straight wire from several sources, in side-by-siderelation in one direction along a path bending the intermediate wireinto zig-zag form while driven along such path and then welding theapices to the flanking side wires to complete the truss structure.

Still another arrangement is found in U.S. Pat. No. 3,641,303 whichillustrates a method and apparatus for continuously producing a trusselement in successive unit lengths by shaping an extended length ofsuitable strip material to present a zig-zag configuration, while in aposition adjacent a chord members provided with integral ribs formedthereon, then welding the web and chord together. Each chord presentsribs of relatively small cross-sectional area which flow under the forceand heat generated by the clamped electrodes of a suitable resistancewelding machine, and thereby form the welds which join the web to thechord member.

OBJECTS OF THE INVENTION

The principal object of this invention is to provide improvements in theelectrical resistance method of welding structural steel members or thelike, wherein electrodes are clamped over the members in the region ofthe weld zone and electrical energy applied thereto; and, moreparticularly, to provide a welded joint of greater quality and strength.

More particularly, it is an object to provide an improved method forcontrolling the application of electrical energy to the work pieceswhereby the useful heat generated by the current and the resistivity ofthe work pieces is more finitely localized in the weld zone itself andwithout appreciable heat loss by conduction of heat away from the weldzone to the more remote regions of the work pieces.

Still another object of this invention is to provide an improved methodas indicated in which the heat generated by impressing an electricalcurrent through the abutting work pieces and within the weld zone ismore uniformly distributed within each work piece.

Still another very important object resides in providing an improvedelectrical resistance welding method that can be adapted tosubstantially balance the generation of heat within each abutting workpiece in the region of the weld zone, over a wide range of work piecesizes.

A further object of this invention is to provide for an improved methodas described for fabricating trusses, and more particularly to thefabrication of open-web steel joists having improved load bearingcapabilities.

Yet another object resides in providing for an improved method forfabricating open-web steel joists wherein the quality and strength ofeach welded joint therein is substantially the same.

It is also an object of this invention to provide a safer method forelectrically resistance welding structural steel members whereinsparking at the weld zone during welding is substantially reduced.

A further important object resides in providing an improved method asindicated which is susceptible of implementation and control throughautomated equipment for the most part in order to relieve the electricalresistance welding machine operator from the demands of constantsupervision.

Still another very important object resides in providing apparatus forcontrolling the improved electrical resistance method for weldingstructural steel members or the like, and more particularly to equipmentcontrol devices for automatically controlling the fusion of the abuttingwork pieces in the weld zone.

Another object of this invention resides in providing equipment forsubstantially automating the fabrication of structural steel memberssuch as open-web steel joints in order to increase the rate ofproduction and decrease the cost of manufacture of same.

It is also an object of this invention to utilize electrodes havinglonger effective wear characteristics in order to substantially reducethe frequency of replacing same.

It is yet another object of this invention to provide improvements inequipment for testing the adequacy of the welds fabricated in accordancewith this invention.

FEATURES OF THE INVENTION

The principal feature of this invention resides in providing a weldingprocess for joining structural members together along a common region ofcontact by means of fusion through the application of heat generatingelectrical energy to the common region of contact, and more particularlyfor applying to said region of contact several timed controlled impulsesof the heat generating electrical energy in sufficient strength adaptedto incrementially raise the temperature of said members in said regioncontact in stages so as to minimize the escape of generated heat energyfrom the region of contact and effectively weld the members along saidcommon region of contact and substantially reduce the formation of welddefects in the region of contact.

The temperature of each member in the region of contact is substantiallythe same during the application of heat generating electrical energy.The strength of each impulse of electrical energy is also substantiallythe same; and as well the time duration of each impulse of electricalenergy is substantially the same.

More particularly, it is a feature to provide a method for weldingmetallic members having predetermined cross-sectional areas whereby themetallic members are positioned and clamped so as to present at leastone region of contact and then applying to said region of contactseveral controlled impulses of electrical energy in sufficient strengthso as to localize the generation of heat energy in the region of contactand uniformly heat and incrementially raise the temperature of saidmembers in said region of contact in stages for effectively welding saidmetallic members along said region of contact and substantially reducethe formation of metallurgical defects in said region of contact. Thestrength of each successive impulse of electrical energy will depend onthe cross-sectional area of the members in the region of contact.

More particularly, it is a feature to provide a method for weldingstructural members along their common region of contact by applying, fora selected timed interval, a controlled electric current havingsufficient strength so as to generate heat in the members in the regionof contact during a selected timed interval and then interrupting theapplication of electric current for a selected timed intervalimmediately upon the elapse of said first preceding timed interval so asto interrupt the generation of heat in the region of contact during theselected timed interval of interrupting the electric current; and thenrepeating the application of electric current and the interruption ofelectric current for said selected timed intervals respectively,successively, several times, immediately upon the elapse of thepreceding timed interval of interrupting the application of electricenergy, so as to uniformly heat and incrementally raise the temperatureof the members in the region of contact in stages and effectively weldthe members along said common region of contact.

It has been empircally determined that for a certain specified range ofmaterials having certain specified cross-sectional areas tabulated intables 1 and 2, that the optimum time duration of each application ofelectric current is in the order of 0.45 seconds and that the optimumtime duration of each interruption of application of electric current isin the order of 0.5 seconds. Moreover, effective results have beenachieved by repeating the application and interruption of electriccurrent between four and six times. The method outlined above achievesits maximum efficiency when the strength of each electric current duringeach of the several interruption steps is substantially zero.

Still another feature resides in applying the welding process asoutlined to the fabrication of metal trusses having spaced chord membersof generally longitudinal predetermined length and cross-section joinedby at least one supporting member of predetermined length andcross-section. The cross-sectional area of the chord members in theregion of contact is substantially smaller than the cross-sectional areaof the web member in the region of contact. By applying a series of timecontrolled intermittent impulses of electric current to the commonregions of contact the temperature of said members in the region ofcontact is incrementally raised in stages so as to minimize thetemperature differential between the temperature of said members in saidregion of contact and said members and thereby minimize heat loss byconduction and minimize discolouration, warpage, twisting, andundesirable metallurgical changes in said members in the form of largegrain growth in the region immediately surrounding the region ofcontact. The application of several time controlled intermittentimpulses of electric current to the said region of contact is alsoadapted to uniformly heat said members in said region of contact so asto more evenly balance the distribution of heat in said members in saidregion of contact and enhance the quality of weld in said region ofcontact.

Still more particularly, is a feature of this invention to apply thenovel application of electrical resistance welding procedures asoutlined to the fabrication of open web steel joists having two spacedsubstantially parallel chord members joined together by a coplanar webbent into a substantially uniform undulating configuration between theparallel chord members so as to present a series of alternating oppositeapices at the bends adapted to contact one of the spaced chord membersrespectively along regularly spaced intervals longitudinally of thechord members by welding the web member to the chord members at thecommon regions of contact.

The chord members used in the fabrication of open web steel joists areformed by drawing chord forming material through a chord forming stationor machine for straightening, shaping, and longitudinally cutting thechord forming material.

The web member used in the fabrication of open web steel joists isformed at a web forming station or machine which uniformly bends equalsections of a web forming material into an undulating configuration ofgenerally uniform pattern having a series of appices presented by thebent web forming material.

It is another feature of this invention to provide a method forautomating the fabrication of open web steel joists as described byclamping at least one of the appices of the web member to one of thechord members and then applying thereto a series of time controlledintermittent impulses of electric current adapted to incrimently raisethe temperature of the chord and web members in the region of contact instages so as to minimize the escape of generated heat by conduction fromsaid region of contact, and to uniformly heat said members in saidregion of contact so as to balance the distribution of heat therein andthereby effectively weld said members along said region of contact so asto substantially reduce the formation of weld defects in the region ofcontact. The members are then unclamped and the open web steel joist isfed along a welding path for a predetermined distance in a direction soas to present the next following apices for welding to the chordmembers. The clamping, application, and feeding steps are automaticallyrepeated successively, several times, immediately upon the completion ofthe preceding feeding step so as to progressively weld each of the nextfollowing alternating opposite apices presented by the bent web memberto the chord member respectively.

It is a feature of this invention that four adjacent apices presented bythe web member are simultaneously and automatically welded during eachof the several number of repetitions outlined above where two of theadjacent apices are welded to one of the chord members and the other twoopposite adjacent apices are welded to the other chord member.

A further feature of this invention resides in a method for fabricatingopen web steel joists where the joist is fed along the welding path adistance equal to twice the regularly spaced interval between the commonregions of contact along each of the chord members during each of theseveral number of repetitions of feeding as disclosed earlier.

It is a further feature of this invention to clean the surface of thechord and web members at a surface cleaning station, or wheelabrator,prior to welding said members so as to minimize sparking at said commonregions of contact during welding and so as to maximize the weldstrength of said joist at said common regions of contact.

Another feature of this invention resides in providing an electricalresistance welding machine for controlling the steps of said method,that include electrodes adapted to releaseably contact and clamp themembers together and transmit and apply the electrical energy to themember in said region of contact on an intermittent basis; a transformerassociated with the electrodes for supplying the electrical energy, tobe intermittently applied to the members in said region of contact; anda control device for controlling the intermittent application ofelectrical energy to the region of contact for a selected timedinterval. The control device is capable of interrupting the applicationof electric energy for a selected timed interval so as to present aseries of intermittent impulses of electric energy adapted toincrimently raise the temperature of the members in the region ofcontact in stages so as to minimize the escape of heat energy from saidmembers in the region of contact and so as to balance the distributionof heat in said members in said region of contact and therebyeffectively weld the members together at the region of contact.

The control device also includes a first timing device for activatingthe application of the several impulses of electric energy for a limitedtime period at preset selected timed intervals and a second timimgdevice for activating the interruption of electrical energy for alimited period at preset selected timed intervals.

The electric resistance welding machine includes a pair of electrodesadapted to clamp the members together in said region of contact duringwelding. The electrodes include heat exchange means for cooling theelectrodes during welding. As the heat exchange means also have atendency to cool the members by conduction, and particularly cool themember having relatively small cross-sectional area, it is a furtherfeature of this invention to provide two electrodes where one of saidelectrodes is of a material having relatively high electrical resistanceto the transmission of electrical energy and adapted to contact themember having relatively small cross-sectional area, and where thesecond electrode is of a material having relatively low electricalresistance to the transmission of electrical energy and adapted tocontact the member having relatively large cross-sectional area. Thematerials of said electrodes have been chosen so as to increase theeffective life of the first and second electrode during the intermittentapplication of electrical energy to the members in the region ofcontact.

It is another feature of this invention to provide a first electrodehaving a tungsten copper beryllium composition and a second electrodehaving a copper beryllium composition.

More particularly it is a further feature of this invention to provide afirst electrode of coper having a 10W3 designation and a secondelectrode comprised of copper having the RWMA Class 3 designation.

Another feature of this invention resides in a first electrode of amaterial adapted to inhibit the cooling of said member having relativelysmall cross-sectional area by said heat exchange means.

Another feature of this invention resides in providing apparatus forfabricating metal trusses having spaced preformed chord members ofgenerally longitudinal predetermined length and cross-section joined byat least one preformed supporting web member of predetermined length andcross-section which are joined together at a welding station adapted toweld the spaced chord member to the web member at the common regions ofcontact. The chord members have relatively small cross-sectional area insaid region of contact and the web member has a relatively largecross-sectional area at the common region of contact.

More particularly in the preferred embodiment the apparatus forfabricating metal trusses includes four pairs of first and secondelectrodes for simultaneously welding two adjacent apices presented bythe web member to one of the chord members, and the other two oppositeadjacent apices presented by the web member to said other chord member,whereby two pairs of said electrodes are located on one side of aconveyor belt and the other two pairs of electrodes are located on theother side of said conveyor belt.

It is yet another feature of this invention to provide a device fornon-destructively simultaneously testing the shear strength of at leastfour welded joints presented by the metallic truss having two spacedsubstantially parallel chord members joined together by a subtantiallycoplanar web member bent into a substantially uniform undulatingconfiguration between the chord members so as to present a series ofalternating opposite apices at the bends welded to the spaced chordmembers respectively along regularly spaced intervals longitudinally ofthe chord members; said device including; a clamping device forsimultaneously clamping the chord members in at least four positionsadjacent the welded joints, where said clamping device includes a devicefor eliminating the twisting of said welded joints from the planedefined by the web member during said simultaneous clamping; and adevice for simultaneously applying a substantially perpendicular forcerelative said plane defined by the web member, to at least four of saidappices defined by the bent web so as to test the shear strength of atleast four of said welded joints.

These and other objects and features will become apparent in thefollowing description to be read in conjunction with the sheet ofdrawings and tables.

DRAWINGS

FIG. 1 is a partial side elevational view of the open web steel joist.

FIG. 2 is a cross-sectional view of the open web steel joist taken alongthe lines 2--2 in FIG. 1 revealing the cross-sectional configuration andareas of upper and lower chord members and web member.

FIG. 3 is a cross-sectional view of the open web steel joist taken alongthe lines 3--3 in FIG. 1 revealing the lower chord member and enddiagonal.

FIG. 4 is a cross-sectional view of the open web steel joist taken alongthe lines 4--4 in FIG. 1 revealing the upper chord member, end diagonal,and shoe.

FIG. 5 is a cross-sectional view of the open web steel joist taken alongthe lines 5--5 in FIG. 1 and is an alternative to FIG. 4.

FIG. 6 is a block diagram illustrating the various steps in thefabrication of open web steel joists.

FIG. 7 is a top plan view of the stacking and assembly zone for chordsand web.

FIG. 8 is a side elevational view of the stacking and assembly zone forchords and web, illustrated in FIG. 7.

FIG. 9 is a top plan view of the electrical resistance welding station.

FIG. 9a is a partial view illustrating the indexing mechanism.

FIG. 10 is a partial view illustrating side elevational view of eachwelding gun.

FIG. 11 is a partial view of the control panel of the welding sequencecontroller.

FIG. 12 is a graphic illustration of the application of severalintermittent impulses of electric current applied to the chord and webmember in the region of contact.

FIG. 13 is a front elevational view of the panel of the operatorcontrol.

FIG. 14 is a side elevational view illustrating the on line sheartesting machine.

FIG. 15 is a top plan view of the on line shear testing machine of FIG.14.

FIG. 16 is a side elevational cross-sectional view of the shear strengthtesting machine.

TABLES

Table 1 tabulates the design shear strength times 2.5 and times 1.65 forvarious sizes of web and chord members.

Table 2 tabulates the optimum perameter settings for the application ofseveral intermittent impulses of electric current applied to the chordand web members at the common region of contact.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Open Web Steel Joist

Referring to the drawings and particularly to FIGS. 1, 2, 3, 4 and 5,the open web steel joist generally indicated at 2 consists of spacedparallel cold formed upper and lower chord members 4 and 6 respectively,which are resistance welded at panel points R to a continuous zig-zagweb rod 8 so as to present an open web. The panel points R are locatedat the common regions of contact between the chord members 4 and 6respectively and the apices presented by web member 8, and are equallyspaced apart a distance known as the panel length P, which in thepreferred embodiment is 24 inches.

The upper chord member 4 and the lower chord member 6 have a generallychannel shaped cross-section are inversely disposed with reference toeach other as best illustrated in FIG. 2. The web rod 8 has a circularcross-section. The cross-sectional area of the chord members 4 and 6 isdifferent from the cross-section of the web rod 8 in the common regionof contact, namely at panel points R.

The ends of the open web steel joist present end diagonals 10 and shoes12 which are arc welded to the joist 2 as shown in FIGS. 1, 3, 4 and 5.The shoes 12 may be formed from two L-shaped brackets as illustrated inFIG. 4 or alternatively may be generally channel-shaped as illustratedin FIG. 5. End diagonals 10 and shoes 12 and the arc welding of same tojoist 2 do not form part of the invention but have been disclosed forcompleteness.

Upper chord member 4, lower chord member 6, zig-zag web rod 8, anddiagonal 10, and shoes 12 are all fabricated from steel.

FIG. 5 illustrates an alternate configuration of shoe 12.

Formation of Chords

The upper and lower chord members 4 and 6 respectively are formed at achord forming station having a chord forming machine 14 as depicted inthe block diagram of FIG. 6.

The upper and lower chord members 4 and 6 are cold formed in the chordforming machine 14 in a well-known manner whereby flat steel plate orchord forming machine is unrolled from a coil, straightened andsuccessively bent by roller dies to impart the cross-sectional area tothe chord members 4 and 6 and depicted in FIG. 2. The chord members arethen cut to length.

Formation of Webs

The zig-zag web rod 8 is formed at a web forming station having a webforming machine 16 as depicted in the block diagram of FIG. 6.

The web forming machine draws steel rod or web forming material from acoil, descales the oxide coatings of the rod, straightens the rod, cutsthe rod to length and then imparts the continuous zig-zag configurationto the web rod 8 by uniformly bending equal sections of the web formingmaterial into an undulating configuration of generally uniform patternhaving a series of apices presented by the bent web forming material,all of which is accomplished in a well-known manner.

Wheelabrator

Although the chord forming machine 14 and web forming machine 16 aredesigned to descale the chord members 4 and 6 and the web rod 8, not allof the scale is removed. Under these conditions, excessive sparkingoccurs at the panel points R or common regions of contact as the chordmembers 4 and 6 are resistance welded to web rod 8; and the welded jointat the panel points 8 frequently break when a small load is applied tothem.

Accordingly, it is important that the cold rolled upper and lower chordmembers 4 and 6 and web rod 8 be passed through a wheelabrator 18 or acleaning surface station prior to welding. The wheelabrator removes theloose mill scale from the chord members 4 and 6 and web rod 8 in amanner known to the art.

Th wheelabrator 18 leaves the chord members 4 and 6 and web rod 8virtually free of scale so that when the corresponding surfaces of thechord members 4 and 6 are electrically resistance welded to the web rod8 a strong clean weld joint at panel points R is produced, with aminimum amount of sparking being produced at the panel point R duringwelding.

Special Treatment of Chords and Webs

The chord members 4 and 6 and web rod 8 are to be left indoors longenough prior to electrical resistance welding to ensure that thetemperature of the chords 4 and 6 and web rod 8 is between 40° F. and90° F. This is necessary in order to minimize the temperaturedifferential between the weld zone and the surrounding metal when thetruss elements are welded at the common regions of contact or panelpoints R.

The time lag between the wheelabration operation and the electricalresistance welding should be short enough to ensure that the chords 4and 6 and web rod 8 remain relatively free of dirt or oxides prior towelding. This time lag should not be more than 5 days.

Stacking and Assembly Zone for Chords and Webs

Once the chord members 4 and 6, and web rod 8 have been formed in thechord forming machine 14 and web forming machine 16, respectively, anddescaled in wheelabrator 18, they are then stacked and assembled priorto being electrically resistance welded as illustrated by the number 20in the block diagram of FIG. 6

Stacking and assembly zone 20 is more particularly depicted in FIG. 7and 8 which includes a generally vertical chord stacking structure 28having cantilevered shelf members 30 and 32. The upper chord members 4are stacked horizontally on lower shelf mmbers 30, while the lower chordmembers 6 are stacked horizontally on the upper shelf member 32. Theoutwardlly projecting end of the upper shelf members 32 are declinedtowards the assembly line 34 (which defines the welding path) in orderto facilitate the manual placement of lower chord members 6 onto theassembly line 34.

The undulating or zig-zag web rods 8 are vertically supported on aseries of hydraulic web supports 36 which are disposed parallel to oneanother at right angles to the assembly line 34 as illustrated in FIGS.7 and 8. The hydraulic web supports 36 include upper horizontal railmember 38 which supports the web rods 8 at the apices of the zig-zagconfiguration. The horizontal rail member 38 is declined at one endthereof to facilitate the manual loading of the web rods 8 onto finger42. The hydraulic web supports are also provided with a stop 40 whichprevents the web rods 8 from falling off the hydraulic web supports 36.

The web rods 8 are manually and individually pulled along horizontalrail members 38 onto finger 42. The finger 42 is connected to ahydraulic unit 44, which when activated, causes the finger 42 to traveloutwardly toward the assembly line 34 carrying the web rod 8.

As the finger 42 travels toward the assembly line 34 the verticallyhanging web rod 8 meets inclined surfaces 46. The inclined surfaces 46are supported along one edge of assembly line 34 as shown in FIGS. 7 and8. As the finger 42 passes over the inclined surfaces 46 the verticallyhanging web rod 8 encounters inclined surfaces 46 and the web rod 8pivots about finger 42, which action causes the web rod 8 to behorizontally deposited on the assembly line 34. The finger 42 is thendrawn back into the dydraulic unit 44.

The upper chord member 4 is then manually placed on the assembly line 34so that it is located intermediate the roller guides 48 and web rod 8.The lower chord member 6 is then manually placed on the assembly line 34so that it is located intermediate the roller guide 50 and web rod 8.The assembly line 34 supports the joist along the welding path definedby the assembly line 34.

The chord members 4 and 6 and web rod 8 are then manually moved to theresistance welding station having electrical resistance welding machine22, which is illustrated in the block diagram of FIG. 6.

GENERAL ARRANGEMENT OF RESISTANCE WELDERS

The electrical resistance welding machine 22 used in the preferredembodiment is known as the Newcor Ser. No. 2055.

At the electrical resistance welding machine 22 the web rod 8 ismanually located relative to the chord members 4 and 6 as illustrated inFIG. 9, such that the assembly is in a web horizontal position. Thejoist assembly 2 is delivered to the resistance welding machine 22 on aconveyor belt 34 which passes through the center zon of the electricalresistance welding machine 22.

The joist chords 4 and 6 and intermediate web rod 8 are clamped in thefirst drive clamps 50 and mechanically propelled forwardly, into theelectrical resistance welding machine 22 a variable amount toaccommodate the initial set up, or an indexed amount established by thedistance between panel points R, or the common regions of contact.

The electrical resistance welding machine 22 in the preferred embodimentcomprises of four welding heads 51 mounted in pairs on guns A and B.Guns A are located on one side of the conveyor belt 34 while guns B arelocated on the other side of the conveyor belt 34. The two welding heads51 on a gun set are mounted 24" apart to match the panel spacing P ofthe joist 2. Guns B are set 12" downstream of guns A to accommodate theoffset between the top chord 4 and the bottom chord 6 panel points R.The distance between the guns A and B perpendicular to the direction ofjoist travel can be adjusted to allow for variation in the joist depthD.

Each pair of guns is electrically controlled by a welding sequencecontroller 60, more particularly identified in the trade as a NENA 12N3H non-synchronous control which shall be described herein. Each pairof guns can be operated independently electrically so that either theupper chord member 4 may be resistance welded to the undulating web rodmember 8 at the two panel points R, or the lower chord member 6 may beresistance welded to the undulating web rod 8 at another set of panelpoints R or common regions of contact. When both pairs of guns A and Bare operating, the upper chord member 4 and lower chord member 6 areeach electrically resistance welded to the web rod 8 at two commonregions of contact. All four guns operate from a common hydraulic source52 in terms of positioning and pressure associated with welding.

Figure 10 illustrates the side elevational view of a typical weldinghead 51.

Each welding head 51 is electrically connected to a 250 KVA single phasewater cooled transformer 59. The primary voltage used is 550 volts. The250 KVA high voltage low amperage of 550 volts is stepped down by thetransformer 59 to a low voltage high amperage output. This output orsecondary voltage may be selected from a series of eight taps byoperating the tap switch 61. There are eight tap settings with secondaryvoltages increasing uniformly from 6.11 volts at the first tap to 14.85volts at the eight tap. The secondary voltage outputs corresponding tothe eight taps are as follows:

    ______________________________________                                                Tap  Volts                                                            ______________________________________                                                1    6.11                                                                     2    7.36                                                                     3    8.61                                                                     4    9.86                                                                     5    11.10                                                                    6    12.35                                                                    7    13.60                                                                    8    14.85                                                            ______________________________________                                    

The welding head includes two electrodes 55 and 57 which are adapted toclamp and weld the lower chord member 6 or upper chord member 4 to webrod 8 at the common regions of contact. The electrodes 55 and 57 arenormally separated when the electrical resistance welding machine 22 isin the non-welding mode. However, when the resistance welding machine 22is operating the hydraulic source 52 activitates the electrodes 55 and57 and forces them together so as to clamp and hold upper chord member 4or lower chord member 6 to web rod 8. Current then passes through theelectrodes and the weld zone and electrically resistance welds the chordmember 4 or 6 to web rod 8. The hydraulic pressure to the electrodes 55and 57 is then removed so that the electrodes 55 and 57 clear the chordmember 4 or 6 and web rod 8.

Each welding head 51 is adapted to pivot about a shaft 53 in thedirection of arrow M so that the welding head is either in the lowerhorizontal position illustrated in FIG. 10, or in an inclined positionsuch that the elecrodes 55 and 57 are located above the joist 2.

The welding heads 51 weld in the horizontal position. The two weldingheads 51 on guns A weld the upper chord member 4 to the undulating webrod 8 at two panel points R at the same time that the welding heads 51on guns B weld the lower chord member 6 to the undulating web rod 8 atanother set of panel points R. Once the upper and lower chord members 4and 6 respectively are welded to rod 8, the welding heads 51 pivot aboutshaft 53 in the direction of arrow M so that the electrodes 55 and 57clear the joist 2, and the joist 2 is then indexed along the conveyorbelt 34 by the first drive clamp 50, a distance of 48" so that the nextpanel points can be welded.

The welding, pivoting and indexing action is repeated until the entirelength of the joist 2 has been welded at panel points R. The indexing isfirst accomplished by the action of the first drive clamp 50, and thenby the action of the second drive clamp 54 alone.

The joist 2 is guided along the conveyor belt 34 by the first driveclamps 50, second drive clamp 54, and a series of guides 56.

The various steps of welding, pivoting and indexing are controlled bythe main operator control panel 58 which activitates a series of relaysin panel 61 in a manner characteristic of the Newcore Ser. No. 2055which is well known in the trade.

INDEXING

The indexing action may be best understood by referring to FIG. 9A whichillustrates the first drive clamp generally depicted as number 50. Thefirst drive clamp 50 includes pinch rolls 50p and drive rolls 50d whichco-act so as to pinch the upper chord member 4. The drive rolls 50d areconnected to the hydraulic motor 50m by a shaft 50s. The drive rolls 50dare activated by the hydraulic motor 50m which is connected to thehydraulic power source 52 by conduit 50c. The hydraulic motor 50m causesthe drive rollls 50d to rotate in the direction of arrow G which drawsor indexes the upper chord member 4 through the electrical resistancewelding machine 22 in the direction of arrow H llustrated in FIG. 9.

A first encoder 50e is connected to the drive rolls 50d by shaft 50s. Asthe drive rolls 50d revolve, the encoder 50e electronically counts thenumber of revolutions of the drive rolls 50d and emits electronic pulsesthrough an electrical conduit 50w which are received by a positioningcontroller 140.

The positioning controller 140 includes a digit display counter 142which shows the number of pulses counted during the indexing of joist 2through the electrical resistance welding machine 22.

In the preferred embodiment a digit count of 3465 represents that thejoist 2 has travelled a distance of 48 inches, or two panel lengths P.Once the digital display counter 142 reaches the count of 3465 thehydraulic fluid from the hydraulic source 52 is no longer supplied tothe hydraulic motor 50m; the hydraulic motor 50m stops and the indexingof joist 2 is complete. The positioning controller 140 includes an indexthumb wheel switch 144 which is used to preset the distance the joist 2will index forward in the electrical resistance welding machine 22; inthe preferred embodiment the index thumb wheel switch 144 is set at3465.

In order to ensure that the joist 2 will stop at the indexed positioncorresponding to the 48 inch distance of travel it is desirable that thejoist 2 begin to slow down near the end of its indexed travel. Thepositioning controller 140 includes a deceleration start switch 146 forthis purpose, which is set at 3100.

When the digit display counter 142 reaches the count of 3100 anelectrical signal is sent through the electrical conduit 50x to adeceleration hydraulic valve 50v which gradually slows down the supplyof hydraulic fluid to the hydraulic motor 50m.

The second drive clamp 54 functions in an identical manner as that ofthe first drive clamp 50, and includes a second encoder 54e which iselectrically connected to the positioning controller 140. A second driveclamp 54 is needed to index the joist 2 through the electricalresistance welding machine when the joist 2 has moved past the firstdrive clamps 20 yet still in the electrical resistance welding machine22.

ELECTRODE COMPOSITION

Electrodes usually include a heat exchange means adapted to cool theelectrodes during welding; and normally comprises of water flowingthrough the electrodes.

A key factor in the economics of manufacturing open web steel joists 2by the multiple impulse welding procedure described herein lies in theutilization of elecrodes 55 and 57 having a specified coppercomposition.

In the preferred embodiment the electrode 55 which comes into contactwith either the upper chord member 4 or lower chord member 6 has arelatively higher electrical resistance to the flow of electric currentthan electrode 57 which comes into contact with the web rod 8. Moreparticularly electrode 55 has a copper beryllium tungsten compositionand the electrode 57 has a copper beryllium composition. The electrode55 used in the preferred embodiment is comprised of copper having a 10W3 designation which is well known in the trade. The electrode 57 whichcomes into contact with the web rod 8 is comprised of copper having anRWMA class 3 composition.

When the 10 W3 and the RWMA Class 3 copper is used in electrodes 55 and57, respectively the electrodes 55 and 57 last much longer thanelectrodes manufactured from other materials or copper compositions, andaccordingly the multiple impulse resistance welding procedure describedherein becomes financially attractive.

It is not known why the electrodes 55 and 57 last longer if they havethe composition described. However, it is known that the chord member 4or 6 has a smaller cross-sectional area than web rod 8 in the commonregion of contact and therefore the heat generated in the chord member 4or 6 has a tendency to be conducted away by the electrode 55 by the heatexchange means referred to earlier. It is suspected that the higherresistance of electrode 55 causes the electrode 55 to heat up to ahigher temperature than electrode 57 and therefore tends to inhibit theconduction of heat from the chord member 4 or 6 in the common region ofcontact by setting up a thermal barrier, which increases the efficiencyof the welding process.

MULTIPLE IMPULSE ELECTRICAL RESISTANCE WELDING TECHNIQUE

The method for controlling the heat balance in welding materials havingdissimilar cross-sectional areas is accomplished by utilizing multipleimpulses of electric current which more uniformally heat thedissimilarly thick masses and result in a stronger weld with lessdiscolouration and warpage than found in conventional techniques.

According to the preferred embodiment of this invention the novel methodfor welding open web steel joists embraces repeated cycles havingessentially the following characteristics.

The welding electrodes 55 and 57 are activated by the hydraulic source52 so as to clamp the chord member 4 or 6 and web rod 8 and exert anextended force on the web rod 8 and upper and lower chord members 4 or 6respectively along the common regions of contact at panel points R so asto urge chord member 4 or 6 and web rod 8 together at the common regionof contact. The web rod 8 and upper or lower chord members 4 or 6respectively are subjected to the electrode force for a short period oftime before the initial application of current to the electrodes. Theinterval of time between the initial application of the electrode forceon the work and the first application of welding current is referrd toherein as "squeeze time".

The electrode force is maintained throughout the multiple impulsewelding technique.

Once the initial squeeze time has elapsed an impulse of current passesthrough the electrodes 55 and 57 and the weld zone in the vicinity ofthe panel points R, which raises the temperature of the weld zone. Thetime that the current actually flows during any one impulse is referredto herein as the "heat time".

The welding current is interrupted or terminated for a specific "cooltime" which represents the interval between the end of one heat time andthe start of the next.

A second impulse of current is then applied which further increases thetemperature of the material followed by a cool time. This procedure isrepeated several times whereby each impulse of electric current raisesthe temperature of the material uniformly and slowly in stages so as toaccomplish correct heat balance. Since the shortest practical time ofcurrent flow is utilized in each multiple impulse the escape ofgenerated heat energy from the region of contact is minimized therebyreducing discolouration, warpage and undesirable metallurgical changesin the metal immediately surrounding the nugget. Also the temperature ineach member in the common region of contact will be more closelymatched, so as to effectively weld the members.

Since the electrode force is maintained during each of the heat time andcool time, each successive interval of heat time in combination withelectrode force causes the web rod 8 and the upper and lower chordmembers 4 and 6 respectfully to fuse or flow and bond with one anotheralong the common regions of contact or panel points R.

After the last impulse of electric current there is "hold time" whichrepresents the time during which the force of electrodes 55 and 57remains applied, and after which the electrode force is removed.

Once the electrode force is removed the welding heads 51 pivot aboutshaft 53 so that the electrodes 55 and 57 clear the joist 2. The joist 2is then indexed along the conveyor belt 34 by the first drive clamp 50,a distance of 48" so that the next set of panel points can be welded, asheretofore described.

In this way the multiple impulse electrical resistance welding techniquecomprises of a series of time controlled intermittent impulses ofelectric current.

WELDING SEQUENCE CONTROLLER

Each pair of guns A and B is electrically controlled by a weldingsequence controller 60 which utilizes NENA 12 N3H nonsynchronouscontrols and which also controls the multiple impulse procedure. Thewelding sequence controller is a component of the Newcor Ser. No. 2055electrical resistance 60 welding machine 22, which is well known in thetrade.

The welding sequence control panel 60 includes seven variable controldials and four switches which control, on a time basis, the varioussequences of the multiple impulse resistance welding technique hereindescribed. The time basis is associated with each cycle of analternating current having a 60-cycle per second power source. Forexample 300 cycles at 60 cycles per second would correspond to 5seconds.

The "squeeze" control dial indicated at 62 is a variable controlallowing setting of the time interval between the initial application ofthe electrode force on the work and the first application of weldingcurrent. Since this is not crucial to the actual welding, it is normallyset at 120 which corresponds to 2 seconds.

The variable "Heat" control dial 64 allows the operator to set andcontrol the interval of time that the current actually flows during anyone pulse. A heat time of 27 for example would represent 27 alternatingcurrent cycles or the equivalent of 0.45 seconds based on the 60 cycleper second power supply, as shall be described herein the heat time isnormally set at 27 for the range of web rod diameters and chordthicknesses covered in Table 2.

The cool time is controlled by the "Cool" control indicated at 66 inFIG. 11, which is the variable control for controlling the interval incycles between the end of one heat time and the start of the next. Inthe preferred method, the cool time is normally set at 3 whichrepresents 3 alternating current cycles or the equivalent of 0.05seconds based on the 60 cycle per second power supply.

The "Hold" variable control dial 68 allows setting of the time intervalin cycles during which the electrode force remains applied after thelast impulse of current. In the preferred method the hold time is set at60 cycles or the equivalent of one second.

The "Weld Interval" variable control dial 70 can be used to set thetotal time interval which includes the squeeze time, all the heat andcool times when making one multiple impulse weld, and the hold time.

The "Off" variable control dial 72 is used to set the length of time incycles when the electrodes 55 and 57 are off the work prior to the startof the next weld. Since the time required to index the joist forward forthe next set of welds actually controls when the next weld can bestarted the off time has no particular meaning and can be set to aminimum time.

FIG. 12 graphically illustrates the current output from the weldingsequence controller 60 during the multiple impulse electrical resistancewelding procedure. The magnitude of the current is found on the ordinateaxis and the time reference is found on the abscissa axis in FIG. 12.

The graph in FIG. 12 demonstrates that there is no current during thesqueeze time when the electrodes 55 and 57 squeeze the work at panelpoints R. But after a short time interval an impulse of alternatingcurrent passes through the electrodes 55 and 57 and the weld zone for aperiod of time known as the heat tine. The resistance of the chords 4and 6 and the web rod 8 to the current causes the weld zone to heat up.After the first burst of alternating current there is a cooling intervalor a cool time when no current is applied. Successive impulses ofalternating current are applied and each are followed by a cool time.The final impulse of alternating current is followed by a hold timewhich is the time interval in cycles during which the electrode forceremains applied after the last impulse of current.

Referring again to FIG. 11 the "Percent Heat" variable control dial 74in conjunction with the tap settings on transformer 51 controls themagnitude of the secondary current used for welding. For example if tap2 is selected by the tap switch 61 and the "Percent Heat" control dialis set at 50% the secondary voltage corresponding to this setting is(7.364.5) 3.68 volts.

The "High Low" switch 75 is operable to engage the high scale mode orthe low scale mode of the "Sequence" control dial 62. Therefore if thehigh scale mode is engaged the squeeze time would be read from the highscale with corresponding higher squeeze time than if the low scale modeis engaged by the "High Low" switch 76.

The "Repeat Non-repeat" switch 78 is operable to control the number ofimpulses of secondary current output. In the non-repeat mode only oneimpulse of alternating secondary current is delivered to the electrodes55 and 57 and weld zone. In the repeat mode the transformer 51 deliversthe programmed number of multiple impulses of alternating current.

The "Spot Pulsation" switch 80 is adapted to put the welding machine 22into a spot welding mode or a pulsation mode. For the multiple impulseelectrical resistance welding technique described herein the "SpotPulsation" switch 80 would be in the pulsation mode.

The "Weld No Weld" switch 82 will eliminate the secondary current in theno weld mode. The "Weld No Weld" switch 2 allows the operator to have apreliminary run through the welding procedure described herein withoutwelding the work so as to insure that the welding machine 22 is workingproperly.

PARAMETER SETTINGS FOR WELDING SEQUENCE CONTROLLER

Table 2 tabulates the settings for the control dials on the weldingsequence control panel 60 for a range of web rod diameter chordthicknesses and joist steps D found in Table l.

For example if a 7/8 inch diameter web rod 8 is welded to an upper andlower chord 4 and 6 respectively each having a thickness of 0.200inches, 750 pounds per square inch of hydraulic pressure is applied tothe weld zone by the electrodes 55 and 57. The "Percent Heat" dial 75would be set at 90% using tap 2; such that the secondary voltage fromthe transformer 51 would be (0.9×7.36) 6.62 volts. The weld intervalwould be set for 330 cycles or the equivalent of 5.5 seconds.

As previously mentioned the squeeze time is normally set at 120 cyclesor the equivalent of 2 seconds. The heat time is set at 27 cycles or0.45 seconds and the cool time is set for 3 cycles or 0.05 seconds.Therefore each impulse cycle which includes a heat and cool time lastsfor 0.05 seconds.

As described herein the hold time is normally programmed for 60 cyclesor the equivalent of 1 second. Since the weld interval equals thesqueeze time plus the hold time plus the product of the sum of the heattime and the cool time multiplied by the number of impulses, the numberof impulses in the particular example is five.

The parameter settings for various sizes of web rod diameters and chordthicknesses may be obtained from table 1 and 2.

DESCRIPTION OF MAIN OPERATOR CONTROL PANEL 58

Although the main operator control panel 58 is an intrical part of theNewcor Ser. No. 2055 resistance welding machine 22 which is common tothe trade, the function for each of the stations on the main operatorcontrol panel 58 depicted in FIG. 13 shall be described.

The "Control Power & Motor On" pilot light 84 indicates that the controlpower is on and is activated by a push button 94.

The "Manual" pilot light 86 is adapted to indicate that the machine isready for operation in the manual mode and is activated by push button96 when operating in a manual mode or by push button 90 followed by pushbutton 96 when switching from automatic mode to manual mode.

The "Auto" pilot light 88 indicates that the electrical resistancewelding machine 22 is ready for operation in an automatic mode and isactivated by push button 98 when operating in the automatic mode or bypressing push button 90 followed by push button 98 when changing fromthe manual mode to the automatic mode.

The "Manual/Auto Reset" push button 90 prepares the electricalresistance welding machine 22 for switching welding machine 22 frommanual and automatic modes of operation and is followed by pushingeither the "Manual" push button 96 or "Auto" push button 98.

The "First Drive Clamp" push button 92 is operable to clamp the upperand lower chord members 4 and 6 respectively of the joist 2 by usingpinch rollers 50p in the first drive clamps 50 so that the joist 2 canbe driven by the drive rolls 50d, into the electrical resistance weldingmachine 22.

The "Control Power & Motor Start" push button 94 is operable to turn onthe electrical control power to the electrical resistance weldingmachine 22 and also starts the hydraulic power source 52.

The "Manual" push button 96 sets the electrical resistance weldingmachine 22 to operate in the manual mode.

The "Auto" push button 98 is operable to set the electrical resistancewelding machine 22 to operate in an automatic mode.

The "No. 2 Encoder Reset" push button 100 is adapted to reset the secondencoder 54e associated with the second drive clamp 54 to 0 which signalrecognizes that the joist 2 has been positioned readily for indexing.

The "First Drive Unclamp" push button 102 is operable to cause the pinchrolls 50p and drive rolls 50d in the first drive clamps 50 to separateand therefore freeing the joist 2.

The "Guns A B" selector switch 104 allows the operator to determinewhich pair of guns will fire when operating in the manual mode.

The "Guns Pivot Advance" push button 106 is operable to cause both gunsA and B to pivot forward in the position shown in FIG. 10 ready forwelding.

The "Index to Preset Creep" selector switch 108 is adapted to advancethe joist 2 to a preset distance established by the encoder when saidswitch is in the index to present mode. The present distance is normallythe amount required to index the joist 2 a fixed multiple of panel pointlengths P. When the "Index To Preset Creep" selector switch 108 is inthe creep mode the joist 2 moves forward at a very slow pace through theelectrical resistance welding machine 22 and does not stop at any presetindex point.

The "Manual Index" push button 110 is operable to cause the joist 2 tomove through the electrical resistance welding machine 22 when operatingin the manual mode. The joist will either advance a preset amount orwill creep along depending upon the position of the "Index to PresetCreep" selector switch 108.

The "Second Drive Clamp" push button 112 is adapted to clamp the upperand lower chord members 4 and 6 respectively of the joist 2 when usingthe second set of drive clamps 54 and is similar in operation to the"First Drive Clamp" push button 92.

The "Guns Advance" push button 114 is operable to cause the electrodes55 and 57 in the welding heads 51 to advance towards each other makingcontact across a panel point R in the joist 2 through the web and chordsection.

The "Guns Pivot Retract" push button 116 is adapted to cause the weldingguns A and B to pivot in the direction of arrow M in FIG. 10 so that theelectrodes 55 and 57 are away from the joist.

The "Index Forward/Reverse" selector switch 118 is used in conjunctionwith the "Manual Index" push button 110 and the "Index to Preset Creep"selector switch 108 to indicate the direction which the joist 2 shouldmove through electrical resistance welding machine 22.

The "Guns Retract" push button 120 is operable to cause the electrodes55 and 57 in the welding head 51 to retract away from each other intothe position shown in FIG. 10.

The "Second Drive Unclamp" push button 122 is used to cause the seconddrive clamps 54 to move away from each other thereby freeing the joist2.

The "A Welder Manual Weld" push button 124 is used to activate the gunset A to weld in the manual mode.

The "A Welder Weld No Weld" selector switch 126 is operable such thatall sequences associated with guns A will activate yet the guns A may ormay not weld depending on the switch position.

The "B Welder Manual Weld" push button 128 is used to activate guns B inthe manual mode.

The "B Welder Weld No Weld" selector switch 130 is operable such thatall sequences associated with guns B will activate yet the guns B may ormay not weld depending on the switch position.

The "Emergency Stop" 132 is used to shut down all functions of theelectrical resistance welding machine 22 immediately.

The "Encoder No. 1 No. 2" selector switch 134 is operable to cause theresponse from either the first encoder 50e or the second encoder 54e tobe displayed on the positioning controller 140.

When the "Single Weld" selector switch 136 is in the off position asingle weld in the automatic mode will be activated by this push button.

The "Weld Repeat Off On" selector switch 138 is operable to cause theelectrical resistance welding machine 22 to operate in the automaticmode but will either limit the number of welds to one in the off mode orallow continuous automatic welding in the on position.

The "Positioning Controller" 140 indicates the distance travelled by thejoist. The counter 142 at the top of the "Positioning Controller 140"shows the number of pulses that the encoder selected by "Encoder No. 1No. 2" selector switch 134 has registered during the movement of thejoist 2 through the electrical resistance welding machine 22. The indexthumb wheel 144 switch determines the total indexing distance for theencoder to preset the distance the joist 2 will move forward in theelectrical resistance welding machine 22. The index thumb wheel 144reflects the distance associated with the "Index to Preset" position onselector switch 108. The decelleration start thumb wheel 146 determinesthe portion of the preset index distance where the joist 2 will begin toslow down in order to stop at the indexed position.

OPERATING SEQUENCE

Before activating the electrical resistance welding machine 22 to weldthe joists 2, the welding sequence controller 60 must be preset to theappropriate weld interval, sequence time, heat time, cool time, holdtime and percentage heat as herein before described. The appropriate tapis selected by utilizing the tap switch 61. The hydraulic pressure isthen selected as outlined in table 2. Tables 1 and 2 outline thesettings utilized in the preferred method for a range of web roddiameters and chord thicknesses. However it should be understood thatthey are not the only settings which can be used in association with themultiple-impulse electrical resistance welding technique.

The index on the positioning controller 140 on the operator controlpanel 58 is then set by programming the index thumb wheel switch 144 andthe decelleration start switch 146 which operation establishes thespecific distance associated with indexing of the welds automatically.

The electrical resistance welding machine 22 is turned on by pushing the"Control Power and Motor Start" push button 94 and then "Manual" pushbutton 96 to set the machine 22 in a manual mode.

The web rod 8 is then locaated in the proper position relative to theupper chord member 4 and lower chord member 6, and then the ends of eachchord member 4 and 6 respectively adjacent to the guns A are checked toensure that they are in the required position. The joist 2 is thenmanually moved past the first drive clamp 50 and the "First Drive Clamp"push button 92 is used to clamp the joist 2 in position. The "SecondDrive Unclamp" push button 122 must be energized to make sure the seconddrive clamp 54 is unclamped for otherwise the machine 22 will notoperate.

With "Index Forward/Reverse" selector switch 118 in the forward positionthe operator brings the joist 2 into the welding machine 22 using the"Index to Preset Creep" selector switch 108 in the creep mode and the"Manual Index" push button 110 until the first set of panel points R onthe upper chord member 4 are opposite their respective welding heads 51on guns A.

At this point the "Guns Pivot Advance" push button 106 is activatedwhich causes guns A and guns B to pivot forward into the horizontalwelding position. The "Guns Advance" push button 114 is activated whichcauses the electrodes 55 and 57 to advance toward each other therebysqueezing the upper chord member 4 and web rod 8. The "Guns A" selectorswitch 104 is set for guns A and then the "A Welder Manual Weld" pushbutton 124 is activated which causes the welding heads 51 on guns A toweld the upper chord member 4 to the web rod 8 at panel points R.

When "Guns Retract" push button 120 is pushed the guns A and guns Bretract from the joist 2. At this stage "Guns Pivot Retract" push button116 is activated which causes guns A and guns B to pivot upwardlyfreeing the joist 2 for indexing.

The joist 2 is moved forward in the electrical resistance weldingmachine 22 into the second welding position by repeating the sequencedescribed above, namely; by selecting the creep mode on the selectorswitch 108, pushing the "Manual Index" push button 110, and thenselecting the forward mode on the "Index Forward/Reverse" selectorswitch 118. The joist 2 is located such that a second set of panelpoints R on the upper chord member 4 will be opposite their respectivewelding heads 51 on guns A, and the first set of panel points on thelower chord member 6 will be opposite their respective welding heads 51on guns B. The joist 2 will have probably progressed sufficiently intothe electrical resistance welding machine 22 to allow the second driveclamp 54 to be engaged by the "Second Drive Clamp" push button 112.

The welding machine 22 is now capable of operating in an automatic mode;and this is accomplished by pushing the "Manual/Auto Reset" push button90 and then the "Auto" push button 98. The "No. 2 Encoder Reset" pushbutton 100 is activated to reset the second encoder 54e at the startingposition to ensure that the first indexing starts from 0000.

When the "Weld Repeat Off On" selector switch 138 is in the off positionthe operator activates two safety buttons adjacent the welding 22 (notshown) and then pushes "Single Weld" push button 136 causing the weldingmachine 22 to automatically make a single multiple impulse weld. If allsystems appear operational the "Weld Repeat Off On" selector switch 138can be set for the on position causing the welding machine 22 toautomatically continue to index and weld the joist 2 throughout itslength.

ON-LINE SHEAR TESTING MACHINE

Periodically one of the welded joists 2 is manually pulled along theconveyor line 34 into the on-line shear testing machine 24 which islocated at the end of the conveyor belt 34 as illustrated in FIG. 6. Theon-line shear testing machine 24 measures the shear strength of theweld.

The on-line shear testing machine 24 is more particularly shown in FIGS.14 and 15 and has been designed to simulate the actual loadingconditions whereby a lateral force F of 1.65 times the design shear loadis applied to the web rod 8 while the chord member 4 and 6 are held inposition. The lateral force F is applied simultaneously to the apices ofthe welded web rod 8 at four points, two of which are adjacent the upperchord member 4 and the other two of which are adjacent the lower chordmember 6 as illustrated in FIG. 15.

The on-line shear testing machine 24 includes four generally parallelframes 148 each of which have a generally quadrilateral configuration asillustrated in FIG. 14. Each frame 148 has a top frame member 150,bottom frame member 152 and two side frame members 154.

The top frame member 150 includes an upper plate member 156 whichsupports hydraulic cylinder 158 and depending arm member 160. The upperportion of the depending arm member 160 is adapted to pivot about pointw as illustrated by the hidden lines in FIG. 14.

The lower portion of the depending arm member 160 is connected to ahydraulic cylinder 162 by means of an elbow joint 164 which is alsoconnected to side frame member 154. The hydraulic cylinder is activatedby the hydraulic pressure from a hydraulic source 182 which istransmitted through conduits 180 and 174. When the hydraulic cylinder162 is activated the elbow joint 164 assumes a horizontal position andthe depending arm member 160 is formed into a substantially verticalposition as illustrated in FIG. 14. When the hydraulic cylinder 162 isdeactivated the elbow joint assumes a generally "Y" shaped configurationillustrated by the hidden lines in FIG. 15, which action pulls thedepending arm member 160 into the position represented by the hiddenlines in FIG. 14.

The depending arm member 160 includes roller 166 which facilitateslocation of upper chord member 4.

The depending arm member 156 also includes a hydraulic cylinder 168which is connected to a finger clamp 170. The hydraulic cylinder is alsoactivated by hydraulic pressure from the hydraulic source 182 which istransmitted to conduits 180 and 176. When the hydraulic cylinder 168 isactivated the finger clamp 170 is forced into the position illustratedin FIG. 14; and when the hydraulic cylinder 168 is deactivated thefinger clamp 170 returns to the position illustrated by the hidden linesin FIG. 14.

The hydraulic cylinder 162 and 168 are deactivated when manually loadingthe joist 2 into the on-line shear testing machine 24 so that thedepending arm member 160 and finger clamp 170 are in the positionillustrated by the hidden lines in FIG. 14. Once the joist 2 is locatedonto the on-line shear testing machine 24 the hydraulic cylinders 162and 168 are activated by the hydraulic pressure from the hydraulicsource 182 and the joist 2 is clamped against the roller 166 by thefinger claim 170.

Then the hydraulic cylinder 158 is activated by the hydraulic pressurefrom the hydraulic source 182 which is transmitted through conduits 180and 178. The hydraulic cylinder 158 exerts a force F to the web rod 8 atpanel point R.

A pressure squeezing valve 172 insures that the joist 2 is clamped intoposition before the force F is applied perpendicularly against the webrod 8. Table 1 tabulates the value of the design shear load times 1.65at the resistance welded joint connection. The design shear load isobtained by recognizing the fact that the design shear load is governedby the buckling of the compression diagonal, that is the diagonal lengthof the web and the web rod dimensions in a manner well known to thetrade.

An electric timer 184 activates the hydraulic source 182 for a minimumof five seconds, thereby providing a lateral force F for a minimum offive seconds and is taken released. This electric timer 184 can beadjusted to increase or decrease the pressure time. The on-line sheartesting machine 24 is non-destructive and successful if no permanentdeformation is developed.

Each of said bottom frame members 152 include a pivot 153 and a spring155 at either ends of said bottom frame member 154 as illustrated inFIG. 14.

The pivot 153 and spring 155 support the frame member 148.

Since it is probable that not all of the welded joints between the webrod 8 and chord members 4 or 6 lie precisely in the same plane when thejoist 2 enters the on-line shear testing machine 22. In a web horizontalposition, the pivot 153 and spring 155 in each of the frame members 148are adapted to allow each individual frame member to automaticallyadjust to the positioning of said welded joint prior to the applicationof the lateral force F. This action substantially eliminates anyinherent stresses that may be caused when the welded joints are not allprecisely in the same plane; otherwise the joist 2 would have a tendencyto twist under the simultaneous application of the lateral force F tothe four points of application.

The preferred on-line shear testing machine 22 simultaneously tests fourwelded joints, although it is possible to contruct the on-line sheartesting machine 24 to test any number of weld joints. Since the on-lineshear testing machine is located at the end of the conveyor belt 34 itprovides a relatively quick and simple procedure for measuring the shearstrength of the four welds simultaneously.

ULTIMATE SHEAR STRENGTH TESTING MACHINE

The ultimate shear strength of the welded joist 2 are periodicallytested so as to achieve failure loads exceeding the value of the designshear load times 2.5 as tabulated in table 1 .

This test is done on an ultimate shear strength testing machine 25designed by the Ontario Research Foundation and is illustrated in FIG.16.

The web 8 and the chord 4 are cut about three inches from the weld jointto facilitate loading in the testing machine as illustrated in FIG. 16.

The ultimate shear strength testing machine 25 simulates actual loadconditions by applying an actual shear force S to the chord 4 while theweb 8 is firmly held in position. The actual shear force S iscontinuously increased until failure occurs. The failure load ismeasured on the pressure gauge (not shown) and is the ultimate shearstrength of the weld connection. The ultimate shear strength variesaccording to the chord 4 and web rod 8 material and geometry. The loadis applied by a hydraulic cylinder 170 which is operated by an electricmotor (not shown).

                                      TABLE 1                                     __________________________________________________________________________    Top and bottom chord: Material CB-45 (ASTM 607), Yield Stress = 45 KSI,       Ultimate Stress = 60 KSI                                                      Web rod: Material G40.21 50W, Yield stress = 50 KSI, Ultimate stress = 65     KSI                                                                           Design Shear load times 2.5 (Kips) for Ultimate Shear testing                 (Pressure gauge reading), Design Shear load times 1.65 (Kips) for on line     testing                                                                       Web rod Dia. (in)                                                             13 16         7 8         15 16           1                                   Joist                                                                              Chord thk. (in)                                                          Dept (in)                                                                          .124                                                                             .154                                                                             .172                                                                             .124                                                                             .154                                                                             .172                                                                             .200                                                                             .124                                                                              .154                                                                              .172                                                                              .200                                                                              .154                                                                              .172                                                                              .200                        __________________________________________________________________________    22   13.5                                                                             13.5                                                                             13.5                                                                             -- 18.1                                                                             18.1                                                                             18.1                                                                             --  20* 20* 20* 21* 21* 21*                              (716)                                                                            (716)                                                                            (716)                                                                            -- (955)                                                                            (955)                                                                            (955)                                                                            --  (1050)                                                                            (1050)                                                                            (1050)                                                                            (1106)                                                                            (1106)                                                                            (1106)                           9.0                                                                              9.0                                                                              9.0                                                                              -- 12.0                                                                             12.0                                                                             12.0                                                                             --  13.2                                                                              13.2                                                                              13.2                                                                              13.9                                                                              13.9                                                                              13.9                        24   10.9                                                                             10.9                                                                             10.9                                                                             14.6                                                                             14.6                                                                             14.6                                                                             14.6                                                                             19.3                                                                              19.3                                                                              19.3                                                                              19.3                                                                              21* 21* 21*                              (573)                                                                            (573)                                                                            (573)                                                                            (772)                                                                            (772)                                                                            (772)                                                                            (772)                                                                            (1019)                                                                            (1019)                                                                            (1019)                                                                            (1019)                                                                            (1106)                                                                            (1106)                                                                            (1106)                           7.2                                                                              7.2                                                                              7.2                                                                              9.7                                                                              9.7                                                                              9.7                                                                              9.7                                                                              12.8                                                                              12.8                                                                              12.8                                                                              12.8                                                                              13.9                                                                              13.9                                                                              13.9                        26   8.9                                                                              8.9                                                                              8.9                                                                              12.1                                                                             12.1                                                                             12.1                                                                             12.1                                                                             --  15.7                                                                              15.7                                                                              15.7                                                                              20.5                                                                              20.5                                                                              20.5                             (470)                                                                            (470)                                                                            (470)                                                                            (637)                                                                            (637)                                                                            (637)                                                                            (637)                                                                            --  (828)                                                                             (828)                                                                             (828)                                                                             (1082)                                                                            (1082)                                                                            (1082)                           5.9                                                                              5.9                                                                              5.9                                                                              8.0                                                                              8.0                                                                              8.0                                                                              8.0                                                                              --  10.4                                                                              10.4                                                                              10.4                                                                              13.6                                                                              13.6                                                                              13.6                        __________________________________________________________________________     *These loads are obtained by the electrical resistance weld capacity.    

                                      TABLE 2                                     __________________________________________________________________________    Machine                                                                             Toler-                                                                  Parameters                                                                          ances                                                                   __________________________________________________________________________    Pressure                                                                            ± 50                                                                           700                                                                              700                                                                              700                                                                              750                                                                              750                                                                              750                                                                              750                                                                              800                                                                              800                                                                              800                                                                              800                                                                              800                                                                              800                                                                              800                          % Heat                                                                                2 94 95 97 88 90 90 90 94 94 96 96 86 88 90                                 ± 0                                                                  Heat Cycle                                                                          ± 0                                                                            27 27 27 27 27 27 27 27 27 27 27 27 27 27                           Cool Cycle                                                                          ± 0                                                                            3  3  3  3  3  3  3  3  3  3  3  3  3  3                            Weld  ± 30                                                                           290                                                                              290                                                                              300                                                                              300                                                                              300                                                                              330                                                                              330                                                                              330                                                                              330                                                                              330                                                                              330                                                                              330                                                                              330                                                                              330                          Interval                                                                            - 0                                                                     Taps  ± 0                                                                            1  1  1  2  2  2  2  2  2  2  2  3  3  3                            __________________________________________________________________________

What I claim is:
 1. In an electrical resistance welding machine forsimultaneously welding together opposed chord members to a uniformlyundulating web-like structural member or the like along alternatejuxtaposed common regions of contact to define a truss or the likewherein the cross-section of one such structural member substantiallyexceeds the cross-section of the other structural member at each suchcommon region of contact,(a) a plurality of electrode means adaptedsimultaneously releasably clamp said members so as to urge and hold sametogether in truss defining relation and establish at least fouralternate juxtaposed conmon regions of contact, each said electrodemeans including a first electrode of a composition having a relativelyhigh electrical resistance to the transmission of electrical energy forcontacting the member of lesser cross-section and a second electrode ofa composition having relatively low electrical resistance to thetransmission of electrical energy for contacting said member of greatercross-section; (b) transformer means associated with each said electrodemeans for simultaneously supplying electrical energy to said membersclamped between each said first and said second electrodes, and; (c)control means associated with said transformer means for simultaneouslysupplying electrical energy to each said first and second electrodes forselected timed intervals said control means including first timing meansfor supplying electrical energy at preset selected timed intervals andsecond timing means for interrupting the supply of electrical energy fora limited period at preset selected timed intervals so as to generateonly intermittent impulses and thereby simultaneously incrementallyraise the temperature of said members in said aforementioned fourregions of contact in stages to the fusion point whereby the escape ofheat energy from said region of contact is minimized and the quality ofweld maximized.
 2. A welding machine as claimed in claim 1 wherein saidfirst and second timing means include first and second manually actuablemeans respectively, for impressing upon said first and second timingmeans respectively, the timed duration of activation of said applicationof said impulses of electrical energy and the timed duration ofactivation of said interrupting means respectively.
 3. A welding machineas claimed in claim 2 wherein said control means include means forcontrolling the strength of each of said several impulses of electricalenergy.
 4. A welding machine as claimed in claim 3 wherein said controlmeans indicate tining means for activating the clamping of saidelectrode means for a selected timed interval.
 5. A welding machine asclaimed in claims 1 or 2 wherein said first electrode means is comprisedof tungsten, copper or beryllium and said second electrode means iscomprised of copper and beryllium.
 6. A welding machine as claimed inclaims 1 or 2 wherein said first electrode means is comprised of copper1.W3 and said second electrode means is comprised of copper RWMA class3.
 7. A welding machine as claimed in claims 1 or 2 wherein each saidfirst and second electrodes includes heat exchange means for coolingsame.